This invention relates to a device for distributing molten metal ("melt" for brevity) particularly during the initial phase of pouring the melt into a mold cavity, and doing so without creating an undue degree of turbulence sufficient to result in defects in an ingot which is to be formed upon cooling of the melt. By "mold cavity" I refer not only to a mold cavity in a conventional mold but also to an ingot-defining mold cavity generated by an electromagnetic force field which provides an invisible "mold". The severity of the defects formed, the particular categorization of each defect, the impact of the defects, individually and severally, upon the quality of the ingot produced, and the economics of producing the ingot, will depend upon the metal being cast and the particular method used to cast the metal, inter alia.
Molten aluminum is typically cast into ingots or billets. Ingots are typically rectangular parallelepipeds with a cross-section of about 50 cm (20 ins) thick, and generally more than 1 meter (39.4 ins) wide, with a length in the range from about 2 meters to 6 meters. By the term "about" I refer to a dimension or physical property which one skilled in the art recognizes as being equivalent to the dimension or physical property stated. The thickness is limited because an ingot tends to develop non-uniform microstructures as the rate of cooling the melt decreases. Rate of cooling is measured along lines radiating from the center of the cooling mass, and it is evident that the rate is highest at the surface of the ingot, and it will decrease as the cross-section of an ingot or billet increases. The width of an ingot is usually limited to about 2 meters because a rolling mill is not wide enough to roll a wider ingot. Billets are cylindrical having a diameter in the range from about 61 cm (24") to about 2 meters, and a length in the range from about 2-6 meters.
As one might expect, aluminum ingots and billets are cast by aluminum producers who tend to accept proven production practices and grow accustomed to the necessity of scalping an ingot or billet, more or less, to remove surface and subsurface defects such as cracks initiated by oxides generated during a cast, and oxides trapped in and near the surface. Because a 0.5 in (1.25 cm) difference in the thickness of aluminum scalped from an ingot typically represents a loss of 720 lb aluminum, growing pressures in the market place demand that the inherent waste by scalping be minimized.
I observed that a typical aluminum ingot is most heavily scalped near its bottom (the "butt"), that is, where melt first contacted the bottom and sides of the mold. A typical ingot has 20 cm of aluminum scalped off its bottom. Many ingots are formed with "butt cracks" or, "non-uniform butt curl", which may require scalping an even greater amount from the bottom of the ingot. In addition, the typical ingot has about 3 cm scalped around its periphery where it contacted the sides of the mold.
More specifically, because the pattern of flow from a melt distributor, and the rate of flow therefrom are critically related to the quality of a cast aluminum ingot, this invention relates to a melt distributor for molten aluminum which is to be cast either in a vertical direct chill ("DC") casting process or an electromagnetic casting ("EMC") process.
My melt distributor is partitioned so as to form a `first` bag partitioned to provide an inner chamber which functions as another (`second`) bag built into the first (hence familiarly referred to as a "bag-in-a-bag"). It is adapted to be positioned beneath a spout from which the melt is to be cast in a process which requires submerging the spout in a pool of melt formed within the bag-in-a-bag. This minimizes the splashing of melt within and outside the bag-in-a-bag, thus minimizes the generation of oxides formed during the initial phase of the "cast". The process is effective because it diffuses the kinetic energy of incoming melt (through the spout) sufficiently so as to generate opposed low velocity discharges of melt from the bag-in-a-bag in a characteristic flow pattern which produces no observable reverse flow at the rim of the starting block and the walls of the mold cavity.
The generation of oxides immediately after melt is cast to form an ingot, is further minimized by a base plate (which forms the bottom of the mold) referred to as a "starting block", which has a recessed central portion or "center well".
Currently in use among some producers of cast aluminum ingots is a "sock" of flexible glass cloth, disclosed in U.S. Pat. No. 3,111,732 (Class 222/subclass 209) to Schroer et al., which is tightly tied around a spout to seal the mouth of the sock around the spout. The main purpose of the sock is to filter particulate matter from the melt. To accomplish this, the closeness of the weave corresponds to the size of particles to be filtered. The sock is woven from glass fiber and has insufficient open area to pass as much melt as issues from the spout under pressure generated by a head of melt above the bottom of the trough. The sock is surrounded by another bag the function of which is to distribute the melt after it is filtered.
To effect the desired filtration of melt, which is the emphasized purpose of the relatively close-woven or close-knit glass fiber sock, it is essential to generate a sufficiently high head to overcome the restriction of flow through the cloth. Such high head generates a relatively high velocity of discharge from the sock which is specifically designed to operate under pressure by virtue of being tightly tied around the spout.
In my invention, I use a pool of melt to submerge the spout within my bag, and use overflowing melt from the pool to provide an essentially "pressureless" discharge (so termed because it distributes melt without building up significant pressure) which lowers the velocity of melt being discharged from the submerged spout and onto the starting block in a mold cavity. By the phrases "without building up significant pressure" and "essentially pressureless" operating conditions, I refer to the "head" pressure under which melt is discharged from my bag into a mold cavity. The head pressure is defined by the head of melt which may accumulate in the bag.
The '732 patent requires that the sock extend far enough below the spout to contact melt in the mold cavity. In actual casting practice, contacting the sock with molten aluminum lessens splashing of the metal but the sock has a high proclivity to being caught at the surface by the freezing metal at the end of the cast. This is attributed to the distribution pattern of melt being such as to lower the temperature of the melt near the surface, in a typical "pour" at a typical "drop rate", to near the freezing point.
The '732 patent contains no teaching of the criticality of controlling the initial phase of the pour when there is no melt in the mold cavity. There is no teaching how to position the sock relative to the starting block just before releasing melt into the mold cavity; and, none as to what effect positioning the sock might have on the generation of oxides as a function of the velocity of the melt being discharged from the sock in the first minute or two of the casting of a typical ingot.
Again, in actual casting practice, one skilled in the art using a sock, typically starts a cast by maintaining the bottom of the sock vertically spaced-apart a great distance (about 30 cm) from the surface of the starting block to minimize the risk of freezing the sock to the turbulently advancing surface of melt which tends to solidify in the initial phase of casting an ingot. Such premature solidification of the melt in the initial phase tends to occur at the start of the pour, because there is an insufficient build-up of head inside the sock, and relatively low mass flow and low melt velocity through the sock. Such conditions result in the impact of too little melt on a cold starting block, and the high rate of heat loss from the melt to a water-cooled, highly heat-conductive metal starting block causes melt to solidify essentially instantaneously.
The use of a sock, the bottom of which in actual practice is spaced relatively far away from the upper surface of a starting block, splashes turbulently against the starting block generating oxides which become embedded in the surface as the melt solidifies. Moreover, such relatively far away spacing requires a long time before enough melt is introduced in the mold cavity to submerge the sock, the greater the distance, the longer the time required.
Another currently popular melt filtration and distribution device, also made from glass fiber, is a canoe-shaped bag marketed under the name "COMBO.RTM. bag" by Kabert Industries, Inc., Villa Park, Ill., but unlike the sock, is a "pressureless" device. The melt from the spout is filtered and distributed over a designated area within the mold cavity. A more detailed description of this prior art bag is provided hereafter to allow one to visualize its construction clearly.
Still another popularly used melt filtration and distribution device also marketed by Kabert Industries under the brand name of MINI.RTM. bag, is a box-shaped open-top receptacle constructed similar to the Combo bag to avoid the pressurized delivery of melt from a "sock". By "open-top" I refer to a top (or mouth) of the bag which is not tied around the spout, or otherwise constricted so as to allow the pressure of the melt to build up in the bag, but is in open communication with the atmosphere in the mold cavity. A more detailed description of this prior art bag is provided hereafter to allow one to visualize its construction clearly. I know of no other bag in current use among aluminum producers in this country.
Irrespective of which currently available bag is used, the vertically spaced apart distance from the starting block is set at the outset of the cast. The bag is positioned by setting the relative distances between the spout, the bottom of the bag, and the starting block, before starting the cast. To set these distances in a typical actual cast, one provides a vertical distance of from about 7.5 cm (3 inches) to about 9 cm (3.5 in) between the bottom of the spout and the top surface of the starting block which forms the bottom of the mold cavity; and, from about 3.5 cm (1.5") to about 5 cm (2") between the bottom of the bag and the top of the starting block.
Like the sock, neither the Combo bag nor the Mini bag are sufficiently effective to control oxide formation, butt cracks and non-uniform butt curl in an ingot. The reason for poor control with the COMBO bag and MINI bag, relative to the control obtained with the bag-in-a-bag of my invention, is that (a) the low level of melt which is maintained in the bag, exposes the discharge end of the spout to atmospheric oxygen, and (b) the vertical impact of the melt on the bottoms of the bags (referred to as "splash platforms") of either the COMBO bag or the MINI bag generates enough metal splash during the start of the cast, and enough surging as a result of waves, to entrap much air.
Enough turbulence and surface waves generate a high level of oxides which adversely affect the economics of ingot production. Some of the oxides are trapped by the solidifying butt shell and may act as initiation sites for butt cracks. The remaining oxides float out to the surface of the melt accumulating in the mold cavity, promote oxidation of metal below, and grow slowly in thickness until they are entrapped on the surface or in the subsurface of the molten ingot as casting proceeds. Patches of entrapped oxide, especially those at the surface, may cause surface cracks and if the ingot does not crack, may require deeper surface scalping for removal than is normally considered economically acceptable.
I realized that most of the defects of a cast ingot (which defects I sought to avoid) appear at the outset of the cast, namely from the initial phase of the casting process, and once formed, are never successfully negated. Therefore I concluded that, if a major improvement in quality of the ingot was to made, the improvement would have to result from an improvement in the flow rate and distribution pattern of melt from the earliest moments during which the melt is introduced into the mold cavity.
To obtain an optimum pattern from the outset of the cast, I concluded I had to diffuse the kinetic energy of the incoming stream within the melt-distribution bag to minimize turbulence when the melt contacted the starting block; and, in addition, that I preferably should confine the flow of melt during its earliest introduction onto the starting block.
To this end I found that the starting block was most effective if its upper melt-contacting surface was not substantially laminar, or only slightly downwardly inclined from either side of the mold cavity towards the center of the starting block, or vice versa, as are conventional starting blocks, but recessed. The recess may be a channel with precipitous side walls, or a center well cut in the block, as will be described in greater detail hereafter. Such a recess must function to confine the initial flow of melt onto the starting block if the recess is to minimize the formation of oxides. Such a recess is not to be deemed equivalent to the cross-shaped recess conventionally cut into the center of the aforementioned starting block (with slightly downwardly inclined surfaces), to lock the ingot being formed, in position within the mold cavity.
It will be recognized that casting aluminum is to commence with a dry starting block, and particular attention is paid to ensure that a "channel" or "center well" in a starting block is dry before starting a pour. In addition, it is desirable to provide drain holes in the bottom of the channel or center well, as well as in other locations in a starting block to minimize the collection of cooling water on the starting block and prevent "bumping" during a cast. The excellent quality of relatively oxide-free ingot obtained by the process of this invention, particularly when used with a channeled, or "center-well" starting block, is evident when compared to the oxide content of an ingot cast with a conventional starting block.
The purpose of redesigning the foregoing prior art bags was to provide a distribution device which was simple, yet effective to distribute melt in the ingot with a minimum of oxide formation, particularly at the outset of a cast, whether as entrapped oxide films, patches, or other undesirable inclusions.